Time-resolved imaging of pulse-induced magnetization reversal with a microwave assist field.

Siddharth Rao,Mathias Kläui,Andre Bisig,Mohamad-Assaad Mawass,Hyunsoo Yang,Jan Rhensius,Charanjit S Bhatia,Markus Weigand

doi:10.1038/srep10695

Abstract

The reversal of the magnetization under the influence of a field pulse has been previously predicted to be an incoherent process with several competing phenomena such as domain wall relaxation, spin wave-mediated instability regions, and vortex-core mediated reversal dynamics. However, there has been no study on the direct observation of the switching process with the aid of a microwave signal input. We report a time-resolved imaging study of magnetization reversal in patterned magnetic structures under the influence of a field pulse with microwave assistance. The microwave frequency is varied to demonstrate the effect of resonant microwave-assisted switching. We observe that the switching process is dominated by spin wave dynamics generated as a result of magnetic instabilities in the structures, and identify the frequencies that are most dominant in magnetization reversal.

Highlights

Figure 1. | Sample schematic and experimental details of the STXM-X-ray magnetic circular dichroism (XMCD) measurement. (a) The sample consists of elliptically-shaped magnetic Py elements below a current-carrying stripline
The Oersted fields generated due to both the unipolar square pulse and the superimposed microwave signal are along the y-axis, the initial state destabilizing magnetization dynamics is observed primarily along the y-axis
The initiation of the switching has been previously attributed in literature to the formation of instability regions as mentioned earlier and the origin of these instabilities comes from spin wave generation

Summary

Introduction

Figure 1. | Sample schematic and experimental details of the STXM-XMCD measurement. (a) The sample consists of elliptically-shaped magnetic Py elements (red) below a current-carrying stripline. | Sample schematic and experimental details of the STXM-XMCD measurement. (a) The sample consists of elliptically-shaped magnetic Py elements (red) below a current-carrying stripline. The current pulse I(t) through the stripline consists of a 2 ns square pulse superimposed on a 4 ns microwave signal (at 1.8 GHz), as shown in (b). The inset in (b) shows a STXM-XMCD differential image of the Py element, along with the relative directions of the pulse and microwave magnetic fields. (c–h) Time-resolved images of the reversal process in a 4 × 0.4 μm[2] Py element. We present direct time-resolved images of the microwave assisted magnetization reversal process in a micron-sized Py (Ni78Fe22) element using STXM. The element is excited by a microwave signal superimposed on a pulse field due to a square current pulse. Using micromagnetic simulations and subsequent analysis, we show that domains are formed around the foci of the elliptical element due to spin waves and couple with FMR frequencies at the edges of the element giving rise to further spin waves of edge modes and fundamental precession modes

Methods

Results

Conclusion